A writing implement of the above-mentioned type, such as a ballpoint pen, can store a water-based ink or any standard ink. The cap allows excessive drying of the tip to be avoided by covering the latter during periods when the implement is not in use. Sealing relative to the outside can be achieved by means of an annular contact of the tubular wall of the cap on an end of the writing body adjacent to the writing tip. In some cases, the cap comprises an inner cap covering the writing tip only.
However, when the body of the writing implement is coupled to the cap, the air inside the cap is compressed, thus generating a pressure increase inside the cap that may sometimes be noticed by some users. When the cap is removed, the air pressure prevailing inside the cap is suddenly reduced, causing a release and pumping of the ink. As the cap is alternately coupled with and removed from the body of the writing implement during normal use, a significant quantity of ink can be released and risks being wasted, for example by remaining clogged in the baffles of an air vent cavity. The inventors found that this phenomenon could be noticeable whenever a liquid ink is used, whether in a free ink reservoir or also in the case of liquid ink stored in a fiber buffer.
A solution for equalizing the internal pressure in the cap consists of making a hole opposite the opening. The document U.S. Pat. No. 5,441,356 thus provides for an air passage in order to avoid pressurizing the inside of the cap when it is fitted onto a pen body. This passage is formed at the bottom of the cap and closed by a sealing valve combined with a spring. With this type of device, air can exit through the hole just prior to the coupling and the spring makes it possible to close off the air passage when the cap is held fixed on the pen body. Several localized inner protrusions of the cap maintain the closed-off position by engaging with a ring-like projection formed on the outer periphery of the pen body.
A drawback of this type of writing implement is the complexity of the sealing system which requires several pieces, including a spring, Assembly of the cap is then more expensive, limiting the appeal of pens that include such a system.